Monocytes and macrophages have been shown to function in a myriad of host defense mechanisms. The spectrum of monocyte functions has become increasingly important and the heterogeneity among monocytes and macrophages has become more defined. It is not known whether the different monocyte functions are carried out by genetically restricted subsets, analogous to T cells and B cells of lymphocytes, or whether a multipotential single monocyte population is capable of functional differentiation in response to appropriated regulatory stimuli. The ability to study the functional potential of monocytes depends on the ability to isolate homogeneous populations and the ability to assess their functions in a reliable manner. We have recently isolated two subsets of human peripheral blood monocytes by elutriation centrifugation. While they share some properties, they differ with respect to others. We have defined the conditions of Percoll density gradient centrifugation under which the elutriation product can be purified further to yield small monocytes in 80% purity. The small, rather than the large, monocyte population in the peripheral blood are the effector cells which are capable of native cytotoxicity against tumor target cells. Both large and small monocytes are equally active in antibody-dependent cellular cytotoxicity. The purified subsets have been used as immunogens and murine monoclonal antibodies are being developed to subsets-specific antigens. We have developed an animal model using guinea pig peripheral blood and have isolated three subsets of monocytes, two of which are analogous to the subsets found in peripheral blood. The third appears to be a more differentiated cell with some characteristics of a macrophage. We are beginning to study the kinetics of response to inflammation of the monocyte subsets in the guinea pig model. Studies of human monocytes will proceed attempting to define antigenic correlates of function with respect to cytotoxicity. Spontaneous and stimulated differentiation in vitro will be studied to define the ontogeny of these effector cells. The guinea pig model will be used for studies requiring the adoptive transfer of local cells to define both differentiation and function in vivo.